Simultaneous transmission of multiple data streams is carried out in a MIMO communication system that employs multiple (NT) transmitting antennas and multiple (NR) receiving antennas. Signals travel from the transmitting antennas via a plurality of paths, undergoing reflection and scattering before arriving at the receiving antennas. A key feature of MIMO systems is the ability to exploit multipath propagation, turning it into a benefit for the user. One such advantage is the increase of system capacity through the use of spatial multiplexing, typically achieved by transmitting independent data on individual transmit links.
With future technology shifting to accommodate a high speed, more ID-based data service, requirements such as spectral efficiency, system user capacity, end-to-end latency, and quality of service management, need to be satisfied. One of the techniques that play a crucial part in meeting some of these criteria is automatic repeat request (ARQ). ARQ is useful for ensuring fast and reliable delivery.
ARQ is a technique for sending a retransmission request for received packet data upon detection of an error in the received packet data. With the transfer of a large amount of high-speed data, more efficient ARQ techniques are typically used to reduce the number of retransmission requests.
In a typical implementation of ARQ, each packet is associated with CRC (Cyclic Redundancy Check) for the purpose of error detection. At the receiver, the content of each packet is validated through the use of CRC. If the packet content is found to contain errors, the receiver will request a retransmission. In the case of MIMO systems, single CRC data (hereinafter “single CRC”) is simply attached to a packet and this packet is sent using multiple antennas. Since substreams transmitted from different antennas normally experience different link conditions, these antennas have different error statistics. The probability that signals on all antennas have errors is very small, especially when a large number of antennas are employed. Since only single CRC is used, the whole packet has to be retransmitted if errors are detected. Therefore, substreams that have already been correctly received are retransmitted, and throughput is wasted beyond necessity.
A method of reducing unnecessary retransmission is proposed in Non-Patent Document 1. The method proposed makes use of encoders per antenna which make ARQ processes performed in each substream. Using multiple encoders per antenna, it is possible to eliminate the constraint of sharing single ARQ processing by multiple transmitting antennas, and multiple acknowledgement signalings are sent back to the transmitter which decides whether to retransmit the substreams in error. Hence, throughput can be increased significantly, and error-free substreams need not to be retransmitted.
Non Patent Document 1: “Multiple ARQ Processes for MIMO systems”/the IEEE International Symposium on Personal, Indoor and Mobile Radio Communications 2002.